Despite the improvements introduced in the fermentation processes in the last years, they still do not present satisfactory performance with reference to the contamination control, due to the difficulty and the high costs regarding the control of the contamination levels of the raw material which is brought to the mill and processed to form the must to be fermented.
Nowadays, the characteristics of the sugarcane processing in the alcohol production mills typically lead to the occurrence of a bacterial microbiota which usually surpasses 104 cells/ml in the fermentation must. Also, the same must, being a medium favorable to the bacterial growth, carries a varieties of contaminant bacteria such as Lactobacilli and wild yeasts besides the fermentative yeasts Saccharomyces cerevisiae. 
The presence of contaminants may causes a decrease in fermentation productivity and other operational problems. For example, wild yeasts, can compete with the fermentative yeasts, absorbing the sugar in the must to be fermentated without converting it to alcohol.
Additionally, the presence of bacteria, Lactobacilli and wild yeasts in the fermentative must tends to provoke agglomeration of these organisms around the fermentative yeasts, thus producing flocculation and impairing the productive activity of the fermentative yeasts.
It should be further observed that, with the increase of the bacterial and Lactobacilli density in the fermentation must, the organic acid concentrations tend to become excessively high. The high organic acid concentration inhibits the multiplication of the fermentative yeasts, which can reduce the productivity of the industrial plants for about 10-20%. In cases of more severe contaminations, higher decreases of ethanol productivity can be observed.
Heretofore, antibiotics have been widely used to control the fermentation must contamination. However, the efficiency of the antibiotics is normally limited to a certain group of bacteria. For example, although the currently available antibiotics have specific action which is desirable against the Gram-positive bacteria and Lactobacilli, they have the reduced action against the wild yeasts. Thus, the use of the antibiotics does not eliminate the productivity losses resulting from the presence of wild yeasts in the fermentative must because the wild yeasts compete with the fermentative yeasts for the sugar contained in the must to be fermented.
In addition, the antibiotics are of slow action in the fermentative must. This characteristic of antibiotics allows the growth of the wild yeasts, bacteria and Lactobacilli sp during an initial phase of antibiotic action. Accordingly, a high antibiotic load is normally required to control the increasing degree of bacteria and Lactobacilli contamination of the fermentative must. Besides, over time, more and more bacteria develops antibiotic resistance, which may also require high loads of antibiotics to obtain acceptable productivity levels.
Antibiotics are normally expensive, increased antibiotic loads may bring the fermentation cost to an unacceptable level. Moreover, higher loads of antibiotics can also cause allergies and other diseases affecting the operators involved with this phase of the process. Further, there is an increasing demand from the consumers in sub-products resulted from the fermentation process for these products to be ecologically friendly and cause no harm to the human being and the environment. This translates to a demand for increasingly smaller residual amounts of antibiotics in the product.
As a consequence of the limitations and inconveniences related to the use of the antibiotics for controlling contaminants, attempts have been made to develop an agent capable of minimizing the deficiencies commented above, some of which are inherent to the use of high loads of antibiotics, while at the same time being effective in reducing the negative effects of the bacteria, Lactobacilli and wild yeasts presented in the fermentative process.
The use of biguanide compounds as antimicrobial agents for inhibiting the growth of contaminant microorganism canker microbe on plants is known. Brazilian patent application PI 0505795-7 (corresponding to the North-American patent application U.S. 60/640,595), to Ecolab, Inc. disclosed a composition for reducing the population of canker microbe (for example, citrus canker microbes), on a series of objects, such as, citrus plants, fruits, seeds, cut flowers, etc. According to the specification, the antimicrobial composition may include a metal antimicrobial agent, for example, silver ion, and a polymer, for example, a poly(hexamethyl biguanide) (PHMB). This composition has bacteriostatic and/or biocidal action and helps in controlling canker growth by applying the antimicrobial composition in an amount and for a time sufficient to reduce the microbial population.
However, the action of the biguanide compound or poly(hexamethyl biguanide)(PHMB) in the Ecolab composition is obligatorily associated with another inorganic antimicrobial agent, so that it can effectively have the antimicrobial function in the “object”, in certain amounts of said agent in the mixture of the components which form the composition, for reducing the population of plant pathogens. Although the silver-based inorganic compounds are known in the prior art, their use in the formation of compositions for controlling the microbial contamination in alcoholic fermentation processes is economically unfeasible, making the prior art proposal inadequate to be used in alcoholic fermentation media for obtaining ethanol.
Another application of the polyhexamethylene biguanide (PHMB) is described by Elszetin, C. and Morais, M. A., in an article entitled “Polyhexamethyl biguanide can eliminate contaminant yeasts from fuel-ethanol fermentation process” published at J. Ind. Microbiol. Biotechnol., (2008), 35:967-973. In the article, the authors examined the fungicidal activity of the poly(hexamethyl biguanide) (PHMB). Based on the effects of PHMB upon growth inhibition and microorganism kill, evaluated in laboratory cultures and industrial samples, the authors proposed the use of PHMB at 200 mg/l to control the main fuel-ethanol contaminants on an industrial scale.
According to the article, the fungicidal effect of PHMB was tested in cells of Saccharomyces cerevisiae strains (JP1 and P2) and wild yeasts of Dekkera bruxellensis, collected directly from the industrial processes. Although these results demonstrate that the presence of PHMB reduces D. bruxellensis below 1 log (44%) at table 2, page 971, the results also show that fermentative yeast S. cerevisiae PE2 is very sensitive to PHMB at concentrations above 20 ppm. Accordingly, the authors recommend using PHMB in combination with a high-fermentating PHMB-resistant strain. Also the authors recommend using PHMB biocide alone in the pre-fermentation vessels, where yeast biomass is aerated and fed with diluted cane juice, to prevent or reduce any negative effects of PHMB on the S. cerevisiae cells when they are exposed to sucrose and to low amount of ethanol.
While demonstrating the PHMB activity as a biocidal agent in distinct applications, the solutions met so far did not take into account the lethal effect of the proportions/concentrations suggested therein, for both the widely used antibiotics and the poly(hexamethyl biguanide) compound, on the fermentative yeasts S. cerevisiae themselves.
Recently, international patent application WO 2009/001205 A2 disclosed a method of producing fermentation-based products, particularly ethanol, comprising fermenting a sugar-containing medium with yeasts in the presence of an additive, for example, guanidine-based compounds, such as PHMB, alone or in combination with other organic biocides such as aliphatic and aromatic monoaldehydes and dialdehydes etc. to reduce or control a bacterial population in the sugar-containing medium.
Although the available prior art can lead to good results in the microbial control in the fermentation medium and also to a reduction in the residual amounts of antibiotics in the sub-products (dry yeasts), none of the prior compositions or methods lead to an increased preservation rate of S. cerevisiae yeast together with an increased elimination rate of the wild yeast, Lactobacilli and other bacteria in the fermentation medium, in order to allow unexpected productivity rates in fermentation processes.
Another drawback of the known processes results from the use of acid for treating the mixed juice received in the mill and for treating the yeast remaining from the fermentation of a load of must to eliminate the bacteria that remained alive after the fermentative process, thus allowing the yeast to be used in the fermentation of a new load of must. The use of acids has a series of drawbacks, such as the need to provide equipment resistant to the acid attack, increased cost associated with greater amounts of acid needed to treat high loads of bacteria remaining in the yeast being reused, the toxicity of this acid input, and the need to eliminate the acid from the sub-product (dry yeasts) of the fermentative process, particularly when used in the animal ration production.
None of the known alcoholic fermentation industrial processes succeeds in satisfying, in an economically feasible manner, all the new current demands that are related to the efficiency of the fermentative process, to the reduction of the loads of antibiotics to be employed in the process, to the level of residual bacteria and antibiotics in the subproduct (dry yeasts), and also to the reduction of the amount of acid, generally sulphuric acid, used for treatment of the yeast, due to the residual bacteria which can be found therein.